In the Pulp and Paper Industry a very caustic solution called white liquor is used in the digesters for cooking wood chips for the production of pulp. This combination (wood chips & white liquor) is sealed and steam is introduced and it "cooks" under high pressure. After cooking this mixture is "blown" from the digester and goes through a seperation process. This process seperates the pulp from the liquid which is referred to as black liquor. The black liquor contains several by-products --turpentine and "soap" (from which tall oil is made) are two plus residual chemicals and B.T.U. (heat) values. After the by-products are removed, the black liquor contains chemical and B.T.U. solids in the magnatitude of about 10%. The higher the solids the higher the value of the black liquor. In order to recover the values this black liquor is processed through evaporators in which the water is removed and in several stages the solids content are raised to 55%-62% for burning in recovery boilers. In the burning process in the recovery boiler, the heat value is removed for the production of energy and the chemical element form a smelt matter called green liquor. This green liquor drops into a heat sink pool at the bottom of the boiler and is removed for processing through a lime kile for the production of white liquor. Quite frequently, prior to recovery of the values in the black liquor, the black liquor is stored in ponds, pools and lagoons.
These large lagoons contain vast amounts of liquor ranging in solids content from weak 2%-20% to heavy 25%-50%. They may remain in such lagoon for months before limited recovery capacity can reclaim them back into the system (in some cases actually years). As the black liquor settles, the solids drop to the bottom of the pond as they fall out of solution and supension. Also the heavier the solids the faster they fall. This is especially true as the high processing temperatures fall in the lagoon environment. The black liquor solids form heavy thick layers over time and become quite firm. Over several years of operation the lagoons have thousands of dollars of value which is literally locked in. It must be pumpable to process. We have witnessed ponds which have actually filled up with these solids reducing the holding capacity of the lagoon to almost nothing. Removal of these solids can create an environmental problem if placed in a land fill.